The present invention relates to sewing machine speed control systems, and in particular to a digital sewing machine speed control system in which the machine speed is gradually increased during startup periods even if the speed control foot pedal is fully depressed for starting the machine.
Conventional sewing machine speed control systems include a circuit that generates a speed setting signal in response to the amount of depression of an operator-controlled foot pedal. The speed setting signal is typically generated by a magnetic sensor located with respect to a permanent magnet which is mounted for movement with the foot pedal, whereby the depression of the pedal results in a variation in the output of the magnetic sensor.
Copending U.S. patent application Ser. No. 199,704 filed Oct. 23, 1980 (now U.S. Pat. No. 4,386,301) by the same applicants as the present invention discloses a high precision sewing machine speed control system having a digital foot pedal position detector for generating a binary code in response to the amount of depression of the pedal and a read only memory for generating a speed control signal in response to the position indicating binary code.
It is of particular importance to the proper functioning of industrial sewing machines that the sewing machine speed be closely controlled in response to incremental changes in pedal depression. In particular, the rise time characteristic of an industrial sewing machine, which is defined by the time within which the desired speed is attained in response to a full depression of the pedal, is an important factor to the proper functioning of the machine. A typical value of the rise time is 100 milliseconds to attain a speed as high as 6000 rpm. However, because of such strict requirements there arises an inconvenience that the thread is cast off the needle during startup periods and the operator has to start again the machine.